The present invention relates to a paste comprising a vinyl polydiorganosiloxane oil and a reinforcing silica filler, and a method for making such paste. More particularly, the present invention relates to a liquid injection molding(LIM) procedure using a silicone paste paste which is made by adding a reinforcing silica filler to an agitated vinyl substituted polydiorganosiloxane oil at a temperature of about -10.degree. C. or less.
A typical LIM silicone composition is shown by Jeram et al. U.S. Pat. No. 3,884,866, directed to high strength addition-cured organopolysiloxane compositions. There is shown, a multi-component combination of a vinyl-containing polydiorganosiloxane fluid, a hydrogen-containing polydiorgansiloxane fluid, an effective amount of a platinum catalyst, and a reinforcing filler, such as a fumed silica filler. Generally, two component mixtures are pumped and mixed at reasonable velocities using conventional LIM equipment to a preheated mold and rapidly cured to produce a rubber part. Component A, can be a mixture comprising a vinyl-containing polydiorganosiloxane fluid, a silica filler, and an effective amount of a platinum catalyst; component B often consists of a hydrogen containing polydiorganosiloxane fluid, in combination with other ingredients of component A except platinum.
LIM products often present a special challange to users of silicone oils, which preferably have a viscosity of from about 20,000 to 100,000centipoise for easy pumping. However, when filled with high levels of reinforcing filler, such as fumed silica, to achieve optimum physical properties in the final molded product, the silica filler can cause an undesireable increase in the viscosity of the mixture which can intefere with pumping velocity. The expression, "application rate", which means g/min of heat curable silicone mixture delivered through a 1/4 inch nozzle at 90 psi, is often the standard used in the silicone industry.
Existing methods for making fumed silica includes burning a combustible mixture of a silicon halide and an oxygen containing gas to produce primary silicon dioxide particles which form aggregates having an average convex perimeter of about 0.5 to about 1.5 microns. One solution to the problem of viscosity build-up in LIM formulations is to reduce the average aggregate size of the fumed silica before it is blended with the silicone oil. A method for effecting fumed silica aggregate size reduction is shown by Kratel et al. U.S. Pat. No. 3,953,487. For example, SiO.sub.2 aggregates are subjected to mechanical impact, such as "ball milling", in the presence of an organosilazane. Another method of provided fumed silica aggregate having a reduced average size is shown by Rohr et al. U.S. Pat. No. 5,340,560, involving the burning of an organosilicon material, such as a silane, or an organosilane in the presence of an oxygen containing mixture. Alternative procedures for overcoming the inherent problem of avoiding viscosity build-up in fumed silicapolydiorganosiloxane oil blends is to add all of the fumed silica to one of the components, such as component A, which can be worked more intensely, prior to the addition of component B which would be at a lower viscosity, and therefor facilitate the processing of the ingredients in certain instances.
It would be desireable therefor to provide additional techniques for making molded silicone parts capable of satisfying the high tensile and tear strength requirements of LIM fabricators. It also would be desireable to be able to injection mold heat curable blends of silicone oil and fumed silica comprising aggregates of primary silicon dioxide particles having an average convex perimeter of about 0.5 to about 1.5 microns without having to pre-mill the fumed silica.